qt: remove obsolete `qt_intf_t::voutWindowType`

... and only consider video-wallpaper if platform is windows.

We can not set an appropriate voutWindowType during initialization, because it depends on qt-compositor which is evaluated later on.

For example, platform windows might use both dcomp and hwnd window types. In fact none of the compositor integrations care about qt_intf_t::voutWindowType but rather set the valid type themselves.

Fo example, compositor_dcomp already uses VLC_WINDOW_TYPE_DCOMP and compositor_platform already uses VLC_WINDOW_TYPE_HWND, effectively making qt_intf_t::voutWindowType obsolete.